In the course of processes of manufacture of steel products that undergo hot-rolling operations or of intermediate products that undergo thermal treatment, such as annealing, it has long been known that the material becomes coated with a more or less thick layer of products of oxidation. It is therefore necessary, considering the need to obtain a degree of surface finish of the finished product that is smooth and shiny, to removing entirely such oxide layers. This is performed through the well-known processes of pickling, for which mixtures of inorganic mineral acids, such as hydrochloric, sulphuric, nitric, and hydrofluoric acids, either alone or in various ratios between them, are generally used.
In the field of stainless steels, on the basis of the knowledge of the industrial processes today in use, the most commonly employed pickling operation involves the use of a mixture of nitric acid and hydrofluoric acid, the reciprocal concentrations of which in the mixture vary according to the type of plant, the type of steel to be pickled, its surface characteristics, and the geometry of the product to be treated. The process is certainly economic and enables excellent results to be obtained; it presents, however, the very serious drawback of creating problems of an ecological nature that are very important and difficult to solve on account of the use of nitric acid. In fact, on the one hand, vapours of nitrogen oxides of the general formula NOx are emitted into the atmosphere, these vapours being extremely polluting and aggressive in regard to the metallic and non-metallic materials with which they come into contact, and, on the other hand, in the washing water and in the exhausted baths high nitrate contents are reached, which consequently must then be disposed of. The problems of purification both of the NOx present in the air and of the nitrates present in the baths entail major problems in terms of plants and systems, high management costs, and the uncertainty of achieving the results required by current standards and regulations in this connection. In the final analysis, then, the expenditure in terms of investments is difficult to sustain in the majority of industrial plants.
A pickling system that does not require the use of nitric acid is therefore called for by industry, and various proposals, above all in the last ten years, have been made in this connection throughout the world.
Processes Alternative to the Use of Nitric Acid
From a critical examination of the patents for cycles proposed as an alternative to the traditional processes for pickling stainless steel based upon HNO3+HF, which no longer contain nitric acid, and from a critical examination of the main technical literature on the subject, it has emerged that:
The British patent No. 2 000 196 of TOKAI Denka Kogyo envisages the use of a pickling bath consisting of ferric sulphate and hydrofluoric acid: to maintain an adequate concentration of ferric ions during the process, H2SO4 and hydrogen peroxide in a molar ratio of 1:1 are continuously fed in. The patent claims the method for controlling the process by continuously measuring the redox potential of the system, which is to be maintained at values xe2x89xa7300 mV by regulating introduction of H2SO4+H2O2.
The two European patents, which moreover are very similar to one another, EP 188975 and EP 236354 (=WO 87/01739), bearing as dates of priority Jan. 22, 1985 and Sep. 19, 1985, respectively, envisage the use of a pickling solution consisting of hydrofluoric acid (5-50 g/l) and trivalent iron ions introduced in the form of fluorinated complexes, into which air or oxygen is blown continuously. The treatment time is from 30 seconds to 5 minutes, and the temperature ranges from 10xc2x0 C. to 70xc2x0 C. A continuous monitoring of the redox potential is moreover recommended; this potential must be maintained, for the first patent, at between xe2x88x92200 and +800 mV, and, for the second patent, at between +100 and +300 mV, and a possible addition of an oxidant, such as potassium permanganate or hydrogen peroxide, is recommended if it is necessary to increase the potential value. All the tests carried out regard pickling of sheet steel alone.
A substantial step forward has been made with the pickling process described in the European patent 582 121 of the present applicant, which employs a bath of H2SO4+HF containing Fe3+ and Fe2+ ions and operating at a controlled redox potential where the re-oxidation of the Fe2+ ions to ferric ions is obtained by periodic additions of H2O2 to the pickling solution.
The process that is the subject of the present patent application constitutes a technically valid, innovative, and from certain points of view, economically advantageous, evolution of the known processes mentioned above; in particular, it represents an important improvement of the previous patent EP 582 121 of the same applicant.
The process according to the present invention has proved particularly suitable for the pickling of stainless steels of the austenitic, ferritic and martensitic series, duplex steels, superaustenitic and superferritic steels, and Ni-based or Ni/Cr-based superalloys.
The process is based on the use of a pickling bath containing iron ions, HF, H2SO4, chloride anions, and conventional additives of the wetting, polishing and inhibiting-agent types, in which an oxidizing agent is continuously or periodically introduced, which is able to convert Fe2+ ions that form in the pickling process into Fe3+ ions, maintaining the redox potential of the pickling solution at the pre-established value.
The oxidizing agent can be introduced into the bath directly just as it is or in the form of an aqueous solution. The oxidation of Fe2+ to Fe3+ can be performed outside the pickling bath as a separate phase of operation, in particular with the method of electrolytic oxidation, such as the one described by WO.97/43.463; or else, it is possible to use air as oxidizing agent in the presence of a copper salt dissolved in the pickling solution as catalyst.
The basic characteristic of the process is the presence in the pickling bath of chloride anions in a ratio of 0.1 to 10 g/l, preferably 1 to 5 g/l.
The presence of chloride ions at the aforesaid concentration in HF, H2SO4 and Fe3+ ion-based pickling solutions increases macroscopically the rate of pickling of stainless steels. This property is all the more evident and advantageous from the industrial standpoint, the greater the difficulty in removing the oxidized layer from the surface of the material.
The properties of the oxide layer, and hence its ease of removal from a stainless-steel substrate, depend upon numerous variables, the most important of which being the composition of the alloy, the conditions of the forms of heat treatment to which the material is subjected, and the thickness and compactness of the oxide layer.
In industrial practice, the situations presenting greater difficulty are found in the treatment of duplex steels, austenitic steels with high chrome content, and chrome-nickel alloys.
In these cases, it may be found that the only way to achieve an effective pickling result in acceptable times is to carry out, prior to the pickling stage, a stage of mechanical pre-treatment (sand-blasting) or chemical pre-treatment (hot-oxidizing aqueous solutions, such as NaOH+KMnO4) or chemico-physical pre-treatment (oxidizing or reducing molten salts) so as to modify the compactness or nature of the oxide.
In addition, sometimes a preliminary chemical treatment is carried out with an aqueous solution containing H2SO4, HCl, HF and their mixtures.
It has been found, according to the present invention, that the addition of chloride ions to a pickling solution containing sulphuric acid, hydrofluoric acid and ferric ions enables, in addition to the acceleration of the pickling rate on each degree of material, also direct pickling in industrially acceptable times of materials that must normally undergo a pre-treatment, like the ones already described.
The reason probably lies in the action of depassivation that the chloride ion is able to exert on the dechromized layer, thus considerably accelerating the phase of detachment of the oxide scale.
For this reason, the concentration of chloride ions represents a critical parameter of the process and must be carefully controlled and monitored.
The concentration of the chloride ions in the pickling bath can be measured with high precision by titration with silver nitrate in the presence an ion-selective electrode which detects the variation in the concentration of Clxe2x88x92 ions in the solution.
Values of chloride-ion concentration that are too high ( greater than 10 g/l) do determine an increase in the pickling rate, but at the same time considerably accelerate also the speed of attack on the base alloy.
Considering that in the pickling process, above all in wire-rod and tube pickling, the pickling times may in any case be very long (30-180 min), an excessive speed of reaction on the base metal may cause an excessively uneven and corroded, and hence industrially unacceptable, surface.
It has been verified that the increase in pickling rate found according to the present invention is not caused by an increase in the acidity of the system. In fact, the effect of the addition of chloride ions is found to be equivalent both when the addition is in the form of salt (e.g., NaCl) and when it is in the form of acid (HCl). In the case of stainless steels that can normally be pickled without mechanical pre-treatment (sand-blasting) or chemico-physical pretreatment (treatment in molten salts), the process according to the invention is equally interesting in that it in any case makes possible a considerable increase in the pickling rate given the same conditions, or else makes possible the same rate of reaction even if the temperature and/or concentration of free acids, in particular hydrofluoric acid, present in the bath, are reduced; in this way, a reduction in the consumption involved in the process is achieved, with benefits both in economic and in ecological terms.
The pickling process according to the invention is generally carried out at a temperature of between 20xc2x0 C. and 70xc2x0 C., preferably between 40xc2x0 and 60xc2x0 C. The temperature depends to a large extent upon the type of steel and the type of plant; in this connection, of fundamental importance is the possibility of using, upstream of the chemical pickling process, mechanical de-scaling treatments. The basic characteristic factors of the process are illustrated in what follows.
It is very important to carry out efficient agitation of the pickling bath so as to ensure continuous renewal of the pickling solution that comes into contact with the metal surface to be treated. The ideal solution is that of spraying, which, however, is not always practicable. In the case of dipping, the injection of air at an adequate rate is the most effective means for guaranteeing evenness of pickling in the most difficult situations (skeins of wire, nests of pipes, heaped items, etc.). In certain cases, it is important to combine recirculation of the pickling solution with the injection of air in order to renew the solution in areas that are difficult to reach with air, such as the upper face of strips which pass horizontally through the pickling bath or the inner surface of pipes.
The air must be appropriately distributed according to the geometry of the bath and of the material undergoing treatment.
The use of air-liquid mixing systems (e.g., ejectors) enables optimization of distribution of the air inside the solution.
Content of Organic Mineral Acids in the Bath
Both the hydrofluoric acid and the sulphuric acid have various functions; among the most important are those of maintaining the pH of the pickling solution at values lower than 2 and of removing the oxides coming from heat treatment and the possible dechromized layer from the metal surface. Hydrofluoric acid, in particular, performs the function of complexing the Fe3+ and Cr3+ ions in the solution and of depassivating the oxidized material, bringing its electrode potential into the dissolution region.
During the pickling process according to the invention, the concentrations of free hydrofluoric acid and free sulphuric acid are regulated in the following ranges:
H2SO4 (free)=50 to 200 g/l
HF (free)=0 to 60 g/l
In the latter case, the lower limit has anyway a value such that the total Fxe2x88x92 anion present in the solution is at least in a molar ratio of 3:1 with respect to the total Fe3+ present.
The total free acidity, which expresses the total content of free acids present in the solution, is between approximately 1 and 7 g. equiv./l.
By xe2x80x9cfreexe2x80x9d acid is herein meant the acid that does not constitute the anion bound in the of salt or complex with the metal cations present in the pickling solution. The total free acidity consisting of the sum of the two free acids (H2SO4+HF) can be determined simply by acid-base titration of the solution appropriately diluted (preferably at least 1:20) and in the presence of an indicator, such as methyl orange (colour-change pH, 3.1 to 4.4) or bromocresol green (colour-change pH, 3.8 to 5.4). The choice of the colour-change point must be made accurately so as to prevent the formation of hydroxides or basic salts of the metal cations present in the solution (in particular Fe3+ and Cr3+). It has been found that in practice there is no precipitation of basic salts of Fe3+ on account of the presence of Fxe2x88x92 anions. Since when the bath is formed, initially the Fe3+ ion is generally introduced into the pickling bath in the form of ferric sulphate, it should be borne in mind that in the presence of free HF there will take place complexation of the ferric ion by the fluoride anion, with formation of various fluoroferric complexes, the main one of which has the formula FeF3.
This entails a decrease of free HF in the solution and a simultaneous increase of free sulphuric acid, according to the following reaction scheme:
Fe2(SO4)3+6HFxe2x86x922FeF3+3 H2SO4
A similar complexation with HF takes place for the Cr3+ cation present in the pickling solution after a certain period of use.
As regards the xe2x80x9cfreexe2x80x9d sulphuric acid (xe2x80x9cfreexe2x80x9d being understood as defined above), this can be determined on the basis of the electrical conductivity of the pickling solution brought to a pre-established degree of dilution, preferably at least 1:20, and on the basis of an appropriate calibration curve.
The method is based on the fact that the electrical conductivity of the solution containing H2SO4 and HF in practice represents the total amount of the free sulphuric acid, which, being a strong acid, is completely dissociated into H+ ions, whereas the free hydrofluoric acid is mainly present in the form of undissociated HF acid, and consequently makes a negligible contribution to the electrical conductivity.
The concentration of free HF can be determined by means of a measurement of the electrical conductivity carried out on the sample of solution already examined to determine the free H2SO4 as described above, after adding to the sample itself a standard volume of concentrated ferric nitrate solution such as to guarantee complexation of the entire HF present, according to the following reaction:
Fe(NO3)3+nHFxe2x86x92Fe Fn(3xe2x88x92n)++nHNO3
from which it is found that for n moles of HF (substantially non-dissociated) there form n moles of HNO3 (strong acid substantially dissociated into H+ and NO3xe2x88x92), and the resulting increase in electrical conductivity enables the free HF present in the solution to be calculated on the basis of a suitable calibration curve.
The method of electrical conductivity makes it possible to measure, with two determinations, the concentrations of the free H2SO4 and HF acids present in the solution, with a precision of approximately 5%, which is amply sufficient for the management of an industrial process and which can be easily applied in a plant.
Alternatively, the free hydrofluoric acid can be calculated with good approximation by subtracting, from the total free acidity, the acidity that can be attributed to the free sulphuric acid, measured using the conductometric method described above.
The pickling solution contains quantities of sulphate anion and fluoride anion and of iron cation (Fe3++Fe2+) which increase as the use of the pickling solution proceeds. The concentrations of these components broadly range between the following limits:
fluoride anion (total) from 15 to 150 g/l;
sulphate anion (total) from 50 to 350 g/l;
max. total ion (Fe3++Fe2+) 120 g/l, preferably not higher than 100 g/l.
Herein by xe2x80x9ctotal fluoride anionxe2x80x9d is meant the sum of the fluorinated, anions, such as Fxe2x88x92 and HF2xe2x88x92, both free and complexed.
By xe2x80x9ctotal sulphate anionxe2x80x9d is meant the sum of the SO42xe2x88x92 anions or anions derived from them, such as HSO4xe2x88x92 bisulphate anions.
Control of Trivalent Iron and of Redox Potential
The redox potential of the pickling system of the present invention is the main function of the Fe3+/Fe2+ ratio, but also depends upon the concentration of the hydrofluoric acid (this causes a decrease in the redox potential) and of the sulphuric acid (this causes an increase in the redox potential) present in the solution.
According to the state of the art, the redox potential of the solution is used as the main parameter for the management of ferric/ferrous salt-based solutions.
In actual fact, the value of the potential of the Fe3+/Fe2+ pair basically depends upon the ratio between the relative concentrations of the two cations, rather than upon their absolute concentrations.
Since the pickling process is governed by the reaction
nFe3++Me0xe2x86x92Men++nFe2+
if the concentration of Fe3+ is not kept above a critical value (approximately 15 g/l), the pickling reaction is too slow.
Consequently, the following conditions must be met:
Fe3+xe2x89xa715 g/l
xe2x80x83Fe3+/Fe2+xe2x89xa70.2
230 mVxe2x89xa6Eredoxxe2x89xa6800 mV
The value of the redox potential, measured using an Ag/AgCl reference electrode, during the process is regulated in the range indicated according to the material to be pickled and according to the working cycle adopted.
Regulation of the Fe3+/Fe2+ ratio can be achieved in various ways, among which:
addition of stabilized hydrogen peroxide;
addition of oxidants, such as persulphates;
addition of oxidants in the gaseous state, such as air, oxygen-enriched air, and oxygen, possibly in the presence of homogeneous or heterogeneous catalysts, in particular Cu compounds dissolved in the pickling solution;
electrochemical oxidation of the solution in a separate operating phase and recycling of the oxidizing solution in the pickling bath.
Among the chemical oxidants, stabilized hydrogen peroxide is particularly recommended for this use. The stabilizer has the function of decreasing the consumption of hydrogen peroxide by reducing the rate of its decomposition in critical conditions, such as high temperatures, and strong acidity, and high concentration of metallic ions, among which in particular Fe3+ and Cu2+ in the case of treatment of steels containing copper.
The presence of the stabilizer is particularly important when it is necessary to operate with an excess of hydrogen peroxide in the solution, such as in the cases where the aim is to have a stage of surface finishing separate from the pickling stage proper.
There are numerous known stabilizers that can be used. Among these, particularly indicated are, for instance, phenacetin, compounds of the families of glycol ethers and aliphatic acids, and non-ionic surfactants terminally blocked with an aliphatic or aromatic radical and their mixtures.
Additives
The functionality of the process can be improved by the presence of additives to be added to the pickling solution. In particular, surfactant compounds can be used with the purpose both of increasing the rate of penetration of the solution inside the porous structure of the oxide and of rendering the attack on the dechromized layer homogeneous.
In particular, for reasons of chemical stability of the pickling solutions and for a better control of the frothing properties, particularly suited are non-ionic surfactants and their mixtures of the following families:
alkoxylated alcohols with free terminal hydroxyls
alkoxylated alcohols with blocked terminal hydroxyls
alkoxylated amines.
According to their concentrations in the solution, these substances are also able to exert a slight inhibitory effect on the base metal, which contributes to improving the surface appearance of the steel.
Work Cycles
The process according to the invention may be carried out both in a single bath and using a number of successive baths.
In the case of the pickling process in a single bath, since the state of passivation of the material must be normally guaranteed at the end of the pickling process and according to the type of the material itself, the redox potential of the solution must be kept above 350 mV, preferably at least 350 mV, and in any case it must fall within the 300-800 mV range.
The Fe3+/Fe2+ ratio must be kept  greater than 1, and preferably  greater than 1.5.
According to the type of material, sometimes it may be necessary to operate with very high Fe3+/Fe2+ ratios, and possibly also with excess of hydrogen peroxide. The hydrogen peroxide may be kept constantly in excess in the solution or, where possible, may be fed locally in the end area of the bath (where the material comes out), being added to the pickling solution on a final spray ramp directed onto the surface to be treated.
In the case where the work cycle makes it possible to have at least two successive pickling baths, it is preferable to operate as follows:
Stage 1
Pickling with a solution according to the invention in which the potential of the solution is maintained at values equal to or higher than approximately 230 mV, and the Fe3+/Fe2+ ratio is maintained greater than 0.2.
Stage 2
Pickling according to the invention in which the potential of the solution is maintained at values higher than 350 mV, and the Fe3+/Fe2+ ratio is maintained greater than 1.5.
Also in this case, if necessary, the hydrogen peroxide can be fed in excess in stage 2, as described previously.
At the end of stage 1, it is preferable to carry out a high-pressure spraying or a mechanical action (brushing) to remove most of the incoherent oxide scale still adhering to the surface. At the end of treatment, the material is in all cases washed with water. Washing must be thorough and preferably carried out by spraying, in that this enables removal even of possible patinas that have formed during pickling or possible residue that has not detached spontaneously from the surface.
In the cases where it is necessary or preferable to have a surface-finishing solution to improve the appearance of the material or to have a separate passivation stage, the following sequence may be adopted:
Stage 1
Pickling according to the invention, in which the potential of the solution is maintained at values higher than 230 mV, and the Fe3+/Fe2+ ratio is maintained greater than 0.2.
Stage 2
Finishing and passivation treatment in a solution containing preferably:
sulphuric acid or phosphoric acid at a concentration of 20-50 g/l;
stabilized hydrogen peroxide at a concentration of 2-15 g/l;
free hydrofluoric acid 0-10 g/l.
As an alternative to hydrogen peroxide, other oxidants can be used, such as sodium persulphate.
A final washing with demineralized water is able to prevent any staining or the presence of saline residue on the material.